The present invention relates to aromatic polyesters having excellent heat resistance and molding workability.
It has been known that aromatic polyesters obtained by reacting terephthalic acid and isophthalic acid or derivatives thereof with 2,2-bis(4'-hydroxyphenyl)propane or a derivative thereof have excellent mechanical properties such as tensile strength and bending strength, thermal properties such as heat distortion temperature and thermal decomposition temperature and electric properties.
However, the thermal properties of these aromatic polyesters are not always satisfactory from the viewpoint of heat resistance required recently of high-performance resins, while their thermal properties are superior to those of araliphatic polyesters such as polyethylene tere-phthalate.
Particularly, these high-performance resins are used frequently in electric or electronic field and various requirements such as a heat resistance in a soldering bath have been put forth. The above-mentioned, known aromatic polyesters are unsatisfactory to cope with these requirements in the practical field. It is thus demanded to further broaden a working temperature range of the resins.
Detailed investigations were already made on fully aromatic polyesters, particularly polyesters comprising terephthalic acid and isophthalic acid or derivatives thereof and 2,2-bis(4'-hydroxyphenyl)propane [see, for example, W. M. Eareckson, J. Polymer Sci., 40, 399-406 (1959)]. As described above, this type of polyesters can be melt-processed to form transparent moldings having excellent mechanical and physical properties. However, the polymers have a glass transition temperature or softening temperature of lower than 200.degree. C. and the resulting moldings have a heat distortion temperature of lower than 170.degree. C. Therefore, it is difficult to use them in fields wherein a very high heat resistance is required or dimensional stability at a high temperature is demanded.
Aromatic polyesters containing bis(hydroxyphenyl) sulfone as a bisphenol component in place of 2,2-bis(4'-hydroxyphenyl)propane have a glass transition temperature of as high as about 265.degree. C., thus showing an excellent heat resistance. However, these aromatic polyesters cannot easily be molded and the resulting moldings tend to become opaque or milky and are quite brittle and easily cracked even if they are produced under optimum molding conditions (see pp. 401-402 of the above reference).
An object of the present invention is to provide aromatic polyesters having an improved heat resistance that has been attained without deteriorating mechanical or electrical properties.